This page provides a high-level introduction to the Supremacy Clause, and is not intended to be a thorough legal review of relevant case law.
Article VI, Section 2 of the Constitution, known as the Supremacy Clause, establishes that the Constitution and the laws of the United States “shall be the Supreme law of the land.” The Supremacy Clause empowers Congress to preempt or supersede State law. Congress can do so expressly with explicit statutory language or by implication when a Federal law occupies the same field as or conflicts with State law.
In evaluating whether a State law is preempted by a Federal statute or regulation, courts typically start with the assumption that State powers are not superseded by a Federal act unless that is the clear purpose of Congress. When Congress has not expressly stated its intent, courts can infer Congress’ intent to occupy a given field of regulation if it has legislated comprehensively, leaving no room for States to supplement. Similarly, courts can infer “field preemption” if Congress’ act relates to a field where the Federal interest is so dominant that the Federal system can be assumed to preclude enforcement of State laws on the same subject.
Courts can also infer “conflict preemption” when there is a conflict between a State law and a Federal statute or regulation. Courts can identify such a conflict when a State law “stands as an obstacle to the accomplishment and execution of the [Congress’] full purposes and objectives.” When a Court determines that there is a conflict, the relative importance of the State’s interest is immaterial; State law must always yield to Federal interests.
The most important Federal laws that can preempt State regulation of electricity are the Federal Power Act, the Public Utility Regulatory Policy Act (PURPA), and the Clean Air Act.
 Rice v. Santa Fe Elevator Corp., 331 U.S. 218, 230 (1947).
 English v. Gen. Elec. Co., 496 U.S. 72, 79 (1990).
 Freightliner Corp. Myrick, 514 U.S. 280, 287 (1995).